The investigators will test whether an intravenous infusion of saline (salt water) will improve the exercise capacity in patients with postural tachycardia syndrome (POTS).

Inclusion Criteria:

• Meet diagnostic criteria of Postural Tachycardia Syndrome (POTS) 28
• Age between 18-65 years
• Male and female are eligible (although the majority of POTS patients are female).
• Able and willing to provide informed consent

Exclusion Criteria:

• Presence of medical conditions that can explain postural tachycardia (e.g., acute dehydration, medications)
• Pregnancy
• Other factors which in the investigator's opinion would prevent the subject from completing the protocol.
• Patients who are bedridden or chair-ridden